Print Binary Tree Layer-by-layer

Question

Print a binary tree layer-by-layer from top to bottom, and from left to right for each layer.

Solution

Yes, it’s a simple task. We can use breadth-first search, and which means we need a queue.

Example

# node structure 
class bst_node: 
    def __init__(self, value, left=None, right=None): 
        self.value = value 
        self.left = left 
        self.right = right
        
# what to do with a node. 
def visit(node): 
    print node.value, # bfs traversal 

def breadth_first_traversal(root): 
    queue = [root] 
    while len(queue) > 0: 
        node = queue.pop(0) # get the head item 
        visit(node) 
        if node.left: queue.append(node.left) 
        if node.right: queue.append(node.right) 
        
# a binary tree and display how was it before reflection 
node_05 = bst_node(5)
node_07 = bst_node(7)
node_09 = bst_node(9)
node_11 = bst_node(11)
node_06 = bst_node(6, node_05, node_07)
node_10 = bst_node(10, node_09, node_11)
node_08 = bst_node(8, node_06, node_10) 
root = node_08 

# answer should be: 8 6 10 5 7 9 11 
breadth_first_traversal(root)